Stalwart Labs LLC This document specifies an extension to the JSON Meta Application Protocol (JMAP) for Mail to enable sharing of mailboxes between users. Building upon the JMAP Sharing framework defined in [RFC9670], this specification extends the Mailbox data type defined in [RFC8621] with properties necessary to configure and manage access permissions for shared mailboxes. The extension introduces a new capability that indicates server support for mailbox sharing and defines the additional properties required to share mailboxes with other principals, including the ability to control which users may access a mailbox and what permissions they possess.